One of communication protocols defined by IEEE (Instituted of Electrical and Electronics Engineers) is IEEE 1394. A communication scheme defined by IEEE 1394 realizes higher-speed bidirectional communication, compared to a handshake scheme. A device connected by an IEEE 1394 serial bus which is a memory bus model interface can access an address designated by a partner. IEEE 1394 defines the protocols of physical and link layers for wider applications, and does not define detailed protocols for each device.
An SBP-2 (Serial Bus Protocol-2) protocol is defined as the protocol of a transport layer utilizing IEEE 1394 as the physical/link layer. SBP-2 fully exploits the features of an IEEE 1394 memory bus model, and a command receiving side can receive data from a transmitting side at the convenience of the receiving side in accordance with SBP-2. Two devices connected by SBP-2 are a command transmitting side called an initiator and a receiving side called a target, and the initiator takes the initiative to start communication. Log-in operation can be performed by only the initiator, and the target operates in response to an action from the initiator in principle.
According to SBP-2, when the initiator logs in, the initiator creates a block ORB which describes a memory address or the like used to exchange data, and notifies the target of the ORB address. The target reads the notified ORB, and reads out data from the described address, or writes data at the address, thereby realizing exchange of data. The ORB address is queued in the target, and its response is queued in the initiator. The target sequentially processes queued ORBs, and sends back responses to the initiator.
An IEEE 1394.3 standard which defines reverse log-in which prompts the initiator from the target to log in is also proposed as a protocol of the transport layer in addition to SBP-2. In IEEE 1394.3, log-in operation can be executed by only the initiator, and the target operates in response to an action from the initiator in principle. However, the target can perform reverse log-in operation which prompts the initiator to log in.
Since the initiator takes the initiative, the standard causes the initiator to provide various services, and the target designates and receives a desired service.
The IEEE 1394 standard assumes that nodes connected to each other constitute one device. This is because IEEE 1394 inherits, e.g., a SCSI definition which assumes each node as a single device in order to eliminate cumbersome management of a plurality of devices within one node.
For this reason, IEEE 1394.3 defines that one physical node (representing a node connected to a 1394 serial bus; to be also referred to as a physical device) has only one initiator unit directory for defining a logical device (unit) at the node of an initiator. This definition inhibits the initiator from having two or more unit directories. If a given physical device has a plurality of logical devices which constitute an IEEE 1394.3 physical device, and plays a role of an initiator, the relationship between the physical device, the logical devices, and services cannot be expressed by a proper hierarchical structure to another device.
FIG. 19 shows an example of the unit directory. IEEE 1394.3 allows each node serving as an initiator defined by IEEE 1394.3 to have only one unit directory with at least entries as shown in FIG. 18. Services providable by the node are described by a description method defined by IEEE 1394.3 in a feature directory at a location whose address is specified by a feature directory offset contained in the unit directory. The node can announce the providable services to another node via IEEE 1394.3. FIG. 20 shows an example of this format.
When each node which can function as an IEEE 1394.3 initiator has the root directory, instance directory, unit directory, and feature directory of the IEEE 1394.3 initiator, an example of the hierarchical structure is shown in FIG. 21. Note that the description methods of the bus information, root directory, and instance directory are defined by IEEE 1212R, and a detailed description thereof will be omitted.
IEEE 1394.3 inhibits each initiator node from having two or more initiator unit directories. There is no method of announcing to another node that a plurality of instances in the initiator can provide the locations of services via IEEE 1394.3 for the respective instances. If a plurality of instances provide services expressed by the same service ID defined by IEEE 1394.3, which of the instances provides a service represented by the service ID cannot be discriminated. FIG. 22 shows an example of the layered structure of directories at the initiator node in this situation. In this manner, the relationship between a physical device, logical devices, and services in an initiator cannot be expressed by a proper hierarchical structure to another device.
If the role of a physical device is a target, the target can have a plurality of unit directories. Even with a plurality of logical devices, they can be hierarchically expressed using unit directories.
A client which uses IEEE 1394.3 as a lower layer must change its behavior depending on whether an IEEE 1394.3 layer is an initiator or target because the method of expressing a logical device and a service provided by it changes depending on whether the IEEE 1394.3 layer of the device is an initiator or target.